Written XmlDataStoreTest, now to implement XmlDataStore...
DataStore was doing duplicate checking that is already done in ContactManager, so removed that (and changed the test).
Everything is working except for a (suspected) bug in ContactManagerImplTest.  I'm moving helper functions out from there and from XmlDataStoreTest to Helper.ContactManagerHelper.  Eventually this should fix the bug (without duplicating code).
Tidied the tests a little, fixed the bug, everything working!  Still to add javadocs for some private functions and do more tidying, but functionally it is finished :-D